It is well known in the art to provide a window with more than one pane of glass separated by an airspace. Such windows are known as insulating windows or insulated glass panels by virtue of the fact that the air or other gaseous material (argon, helium, nitrogen, etc.) trapped within the space between the glass panes serves as an insulator to reduce heat flow through the glass.
Typically, the glass panes are separated by a spacer frame formed from sections of tubing joined together at adjacent ends to form a continuous frame. The spacer frame lies between the glass panes and extends around their perimeter. The tubes comprising the spacer frame, also known as spacer frame bars, are commonly made of metal, such as aluminum alloy or steel or stainless steel: in addition to being commercially economical, these materials are sufficiently strong and rigid to permit the tubes to function as spacer frame bars. Also, aluminum and steel exhibit good corrosion resistance, and their structural integrity is not adversely affected by long-term exposure to sunlight.
The use, however, of an aluminum or metal spacer frame is not without its problems. A significant heat transfer problem may arise because an aluminum or metal spacer is a much better heat conductor than the surrounding airspace. Because the spacer and glass panes are contiguous, the spacer itself acts as a conduit for energy transfer between inside and outside panes of glass. Thus, significant energy loss may result because of the spacer's physical contact with the glass panes.
One partial solution to heat transfer through the spacer is provided by U.S. Pat. No. 5,568,714 to Peterson. The invention of Peterson provides an elongate tubular spacer with an integral thermal break that reduces energy flow between glass panes. Although the thermal break impedes heat transfer through the spacer, heat transfer impedance can still be an issue because the metal on either side of the thermal break still rapidly conducts thermal energy.
Another partial solution is provided by U.S. Pat. No. 5,377,473 to Narayan et al. The invention of Narayan provides a spacer having a lower web which is generally W-shaped in cross-section to provide a lengthened thermal path, and an upper web which is pierced by a series of staggered slots. The slots eliminate any straight-line thermal path across the upper web, thereby increasing the effective length of the thermal path, and also allow fluid contact between the air/gas in the interpane space and a desiccant material which is encased within the spacer. Unfortunately, the slots in the invention of Narayan et al. also allow the desiccant material (typically, a silica gel or other material which is in a granular or powder form so as to maximize surface area) to escape from the interior of the spacer into the interpane space, especially along the sides and top of the window. Once the desiccant material escapes into the interpane space it tends to collect on the inside surfaces of the glass panes, where it is impossible to remove, thereby giving the window a permanently cloudy or dusty appearance.
Accordingly, there exists a need for an improved metal spacer bar which defines elongate thermal paths between glass panes for enhancing thermal efficiency of insulated windows or other panels. Furthermore, there exists a need for such a spacer bar which establishes fluid contact between the air or other gasses in the interpane space and a desiccant material which is encased within the spacer bar, but without possibility of the desiccant escaping from the spacer bar into the space between the panes.